Project 2 ? Summary/Abstract The primary goal of Project 2 is to conduct a randomized controlled trial (RCT) to evaluate whether folate+B12 supplementation to 8 to 10 year old children can: a) increase arsenic (As) methylation and lower blood As concentrations (bAs) (Aim 1); and b) improve As-related decrements in cognitive function (Aim 2). Ingested inorganic As (InAs) undergoes hepatic methylation to generate mono- (MMA) and di-methyl (DMA) arsenicals using s-adenosylmethionine (SAM) as the methyl donor; this important process facilitates renal As elimination. Methyl groups derived from folate and B12 are essential for the synthesis of SAM through one- carbon metabolism (OCM). RCTs in adults from Dr. Gamble's group (PI of previous Project 3) have determined that folate supplementation increases As methylation and significantly lowers total bAs, particularly blood MMA. However, one cannot assume that folate supplementation would be comparably effective in children because OCM is considerably up-regulated during periods of rapid growth in order to meet the demands for nucleotide and protein biosynthesis, potentially at the expense of methylation reactions. Nutritional interventions are needed to evaluate if folate + B12 can increase As methylation and lower bAs in children, these important questions will be addressed in Aim 1. Research over the past 15 years led by Drs. Wasserman and Graziano (former Project 2) and others has documented significant associations between children's exposure to As and poorer neurodevelopment. In a follow-up study of 10 year-old children in Bangladesh, reductions in As exposure over time were associated with significant improvement in tests of Working Memory. Some nutritional deficiencies have well documented effects on cognitive function in children. However, the influence of folate and B12 deficiency ? nutrients known to be required for proper neural development, neurotransmitter synthesis and myelination (in addition to As methylation) ? on children's cognitive function have received relatively little attention. There is a high prevalence of folate and B12 deficiencies among young children in Bangladesh. Nutritional deficiencies and As exposure may influence overlapping cognitive outcomes, and may exacerbate As-induced decrements in cognitive function. In our preliminary data in children, we find that B12 deficiency is associated with decrements in intelligence (by WISC-IV) in boys, and that folate deficiency is associated with decreased arsenic methylation in both boys and girls. In Aim 2, we will extend our team's previous work on As and cognitive function to test the hypothesis that, in our RCT from Aim 1, folate+B12 supplementation leads to improvements in tests of cognitive abilities, including those previously associated with As exposure, as compared to placebo. Additional aims evaluate other OCM related nutritional parameters and their relationship to As methylation in Bangladeshi children and in young Native American Indians. Positive findings of these studies could have profound implications for the health and productivity of many As-exposed populations.